This month I would like to discuss the need for both over-temperature and high pressure protection for storage type water heaters.

First, it is important to understand that additional (beyond primary from the factory) over-temperature (210F or above) and high pressure (150psi and above) protection are required for all water heaters, whether storage type or tankless (see 2006 IRC 2803.1). Also, it is high temperature that can cause the potential for explosion; thus over-temperature devices protect both persons and equipment. High water pressure has only the potential to cause leaks, not injury; so high pressure devices protect equipment only.

The most common method of both high pressure and high temperature supplemental protection are combination Temperature and Pressure Relief Valves. These valves require installation in the upper six-inches of the tank in a storage type heater and the discharge should be pipes per local requirements. Installation in tankless heaters should be per the manufacturer instructions.

Many jurisdictions allow gas water heaters to be over-temperature protected by a Watts #210 automatic gas shutoff device when piping for a conventional TPRV cannot be installed sloped to drain. These valves are installed again in the upper six inches of a tank type heater and turn off the gas supply in the event the water temperature reaches 210 degrees F. It is important to note these valves only sense water temperature, not pressure. A pressure relief is still required. The Watts Regulator instructions indicated the pressure only relief should be installed near the water heater, on either the hot or cold side. See http://www.watts.com/pro/_productsFull_tree.asp?catId=64&parCat=290&pid=3435&ref=2 for specific installation instructions. Many jurisdictions, realizing that the pressure relief cannot be properly drained if installed at the water heater (the primary reason a Watts 210 is used), allow the pressure relief to be installed anywhere in the water distribution system after the main shutoff valve. Pressure in the water distribution system is uniform throughout when water is not flowing to the valve can react regardless of location.

Expansion tanks are required for most storage type water heaters in modern installations. These tanks prevent pressure increase due to thermal expansion when the heater is operating; they do not protect against over-pressure conditions like a pressure relief valve. Expansion tanks are not required for tankless heaters as thermal expansion only occurs when water is flowing (the only time the heater operates) so there is no pressure buildup concern.

For more information check out Code Check Plumbing, third edition.

We'll talk next month,

Mike Casey
Kaplan Professional Schools
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